Electronic methods for locating transmission line faults include: relating oscillographic readings to short-circuit study data; processing digital oscillograph readings in a fault locating program; and using two-end travelling-way fault locators, one-end travelling-way fault locators, one-end impedance-measuring fault locators, or two-end impedance-based fault locators. The one-end impedance-measuring fault locators calculate the fault location from the apparent impedance seen looking into the line from one end. They have been proven to be the most practical, since no communications channel (other than possibly one for remote reading of the fault location) is required, and they are generally easy to install and operate. Two-end impedance-based fault locators use the voltage and current information at both ends of the line to calculate the fault location. The advantage of this scheme is that ground faults can be located without knowing the zero-sequence impedance of the transmission line. The disadvantage is that data must be retrieved and then processed by a relatively skilled individual.
Locating faults requires many of the same signal processing steps as protecting transmission lines. To accurately locate all fault types in a three phase (A, B, C) system, the phase-to-ground voltages (VA, VB, VC) and the currents (IA, IB, IC) in each phase must be measured. However, when line-to-line voltages (VAB, VBC, VCA) only are available, it is possible to locate phase-to-phase faults accurately. Ground faults can also be located reasonably well in most cases, if the zero-sequence source impedance (ZS0) is known.
The phasor quantities must be extracted. This requires filtering to ensure that transients do not affect the measurement of phasor quantities. Analog and digital filters may be used. The analog filter removes all high frequency components, and the digital filter removes DC offset.
Knowledge of the fault type is essential for accurate single-end fault locating, as the fault type determines the measuring loop to be used. Different techniques may be used to determine fault type. One technique is to determine the fault type from the relay elements which operate. The other technique is to use a separate fault-type determination process exclusively for the fault locator. This latter technique tests and compares the phase and residual currents (IR). Another technique which has been used is to use the information from external starting elements such as the distance or overcurrent elements in a line protection terminal. Still another way, which has been used by programs which analyze digital oscillographic records, is manual specification of the fault type, relying on a skilled operator for fault-type determination.
As is known, various impedance calculations may be employed, depending on the fault type (ground faults, three phase faults, phase-to-phase faults, and phase-to-phase to ground faults) to calculate the apparent positive-sequence impedance (Z1) of the fault. One of the following impedance calculations may be employed, depending on the fault type, to calculate the impedance Z1 to the fault:
Ground (G) Faults:
______________________________________ AG: Z1 = VA/(IA + k .times. IR) BG: Z1 = VB/(IB + k .times. IR) CG: Zl = VC/(IC + k .times. IR) ______________________________________
where k (the residual current compensation factor)=(ZL0-ZL1)/3ZL1, IR=the residual current, ZL0=the zero-sequence impedance of the line, and ZL1=the positive-sequence impedance of the line.
Phase-to-Phase and Phase-to-Phase to Ground Faults:
______________________________________ AB or ABG: Z1 = VAB/IAB BC or BCG: Z1 = VBC/IBC CA or CAG: Z1 = VCA/ICA ______________________________________
Three-Phase Faults:
Any of the above equations.
The measured impedance unfortunately depends on many factors not represented in these equations. These include no or imperfect transposition between the fault and the measurement bus, mutual coupling to nearby circuits, load flow, and fault resistance. Other problems arise from taps, conductor configuration changes, instrument transformer errors, nonuniform or unknown soil resistivity, etc.
Once the apparent positive-sequence impedance Z1 to the fault is calculated, the distance to fault is determined by dividing the measured reactance by the total reactance for the line and multiplying by the line length. This approach, which is a straight reactance calculation, eliminates the effects of fault resistance under conditions of light loading. On more heavily-loaded lines, faults with considerable resistance are not accurately located by this method, since the voltage drop at the fault in the fault resistance has both a resistive and a reactive component, as seen from either end. The reactive component of this drop is an error term not eliminated by this simple calculation. A calculation which takes prefault load flow into account to reduce the effects of fault resistance and load flow on fault location calculations is disclosed in Takagi, et al., "Development of a New Type of Fault Locator Using the One-Terminal Voltage and Current Data", IEEE Transactions on Power Apparatus and Systems, Vol PAS-101, No. 8, August, 1982.
Generally, on radial transmission and distribution lines, or any line where the infeed from the remote end is small compared to the total fault current, or when the load flow is small on an interconnection, the fault-locating errors due to fault resistance and load flow are negligible, even with the straight reactance calculation. This is useful to know, since a hand calculation of fault location used in the straight reactance calculation is easier than a hand calculation used in the above-discussed Takagi, et al. algorithm. Indeed, when pre-fault information is unavailable, the Takagi, et al. algorithm cannot be used.
Tapped loads seldom make any significant difference in fault location, since delta-wye transformer connections are usually used (no ground source), and since the impedances of the transformers are generally large compared to the line impedance. However, the tapped load currents do make a difference when the load current is near the short circuit current.
The method of the present invention provides a means for locating phase-to-ground faults in radial transmission and distribution lines with tapped loads. This method, however, will not locate three phase faults, phase-to-phase faults and phase-to-phase to ground faults. However, about 90% of all faults are phase-to-ground faults.
Thus, an object of the present invention is to provide a method for locating phase-to-ground faults on radial circuits with tapped loads where the load currents are significant.